Powershift transmissions are used in a variety of work machines where it is desirable to shift gears without interrupting the torque flow through the transmission. To this end, transmissions generally have a clutch overlap period whereby both an first speed clutch and a second speed clutch are engaged at the same time. If this overlap period is too short, then there will be a significant torque interruption to an output shaft prior to the engagement of the on-coming clutch. If the overlap period is too long, then the shift will expose the transmission to stresses which can potentially reduce the life of various transmission components. Additionally, transmissions may also have a directional clutch which bears most of the load during a shift between the first speed clutch and the second speed clutch. Therefore, it is also important to control the rate and timing of the engagement of the directional clutch with respect to the speed clutches to ensure that the load is properly carried by the directional clutch during the shift.
A single timing of the disengagement of the first speed clutch, engagement of the second speed clutch, and the disengagement and reengagement of a directional clutch is not satisfactory for all load conditions. In particular, under high load conditions, it is desirable to increase the overlap period between the first speed clutch and the second speed clutch, increase the rate at which the second speed clutch is engaged, and increase the rate at which the directional clutch is reengaged to effectively complete the gear shift. On the other hand, under light load conditions, it is desirable to decrease the overlap period between the first speed clutch and the second speed clutch, and reduce the rate at which the directional clutch is engaged to effectively complete the shift. Moreover, the pressure used to hold and engage the speed clutches and directional clutches may also be varied to improve the shift. Under high load conditions, it is desirable to increase the engagement pressures of the second speed clutch and directional clutch whereas under low load conditions, it is desirable to decrease the engagement pressures of the second speed clutch and directional clutch.
What is needed therefore is a method and apparatus for adaptively engaging speed and directional clutches based on transmission load.